Inop outflow valves and gradual decompression
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Inop outflow valves and gradual decompression
Hi there,
Does it mean that, say if the outflow valves become inop during cruise leading to the gradual decompression of aircraft, the negative/positive relief valves will keep the cabin pressurized?
Thanks
Does it mean that, say if the outflow valves become inop during cruise leading to the gradual decompression of aircraft, the negative/positive relief valves will keep the cabin pressurized?
Thanks
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No - in cruise the valves are almost full closed (flight segment with max diff px) so if they fail in that position and the cabin pressure starts to fall then you cannot increase the pressure by closing that valves further as they are inop. Depressurization will continue until the diff px gets to a point that corresponds with the fixed valve position.
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Thanks for the replies guys.
Can the positive pressure relief valves replace outflow vavles since they perform the same function? I'm just thinking the purpose of those pressure relief valves? I understand the basic undertaking but I'm thinking of scenarios whereby they will come into play operationally although most of the times, it's transparent to us. Problem is FCOM 2 has only 2 lines of it regarding relief valves...
Can the positive pressure relief valves replace outflow vavles since they perform the same function? I'm just thinking the purpose of those pressure relief valves? I understand the basic undertaking but I'm thinking of scenarios whereby they will come into play operationally although most of the times, it's transparent to us. Problem is FCOM 2 has only 2 lines of it regarding relief valves...
The outflow valves are designed to give a controlled pressurization, the relief valves or if part of the same unit will operate at a higher differential to prevent structural damage and likely move rapidly. In the older models testing we would cover their outlet with a caste chamber and evacuate that, to cause the differential, when the valve 'popped' open the units gauge would show the pressure at which it activated; in olden days it was often a problem that nicotine build up would delay this action. Negative relief valves are usually a separate item, very often fitted in the rear pressure bulkhead to allow air into the fuselage in the event of a sudden aircraft decent before the pressurization supply could catch up.
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Using a B.747 as an example ; Both outflows operate independently of each other and the chances of both of them failing completely are almost zero . Don't forget , they can also be toggled manually via both AC & DC motors As BAeng. says , in the crz they are just 'pulsing' and are almost closed anyway.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
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Using a B.747 as an example ; Both outflows operate independently of each other and the chances of both of them failing completely are almost zero . Don't forget , they can also be toggled manually via both AC & DC motors As BAeng. says , in the crz they are just 'pulsing' and are almost closed anyway.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
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To cut a long story short - no
Assuming failure of outflow valves to maintain cabin pressurization in whatever situation due to excess leak or failure to pulsate effectively to maintain pressurization the cabin altitude in a steady FL wil continue to rise in accordance with the amount of leak (either via fuselage or partially stuck open outflow valve)
The positive pressure relief valve is a mechanical device. It has an air filter and two ambient pressure sense connections. The air filter supplies cabin pressure to the valve. The ambient pressure connections supply ambient pressure to the valve
When cabin pressure differential at the remote ambient pressure sense port increases to 8.95 psi, the relief valve opens. This permits air to go out of the airplane. If the remote pressure sense port does not function, the relief valve opens when cabin pressure differential at the integral ambient sense port increases to 9.42 psi.
When cabin pressure differential goes below the limit, the positive pressure relief valve closes
The negative pressure relief vent opens when cabin pressure is less than ambient pressure. This prevents negative cabin pressure. Too much negative cabin pressure could damage the fuselage structure.
The negative pressure relief vent is a spring-loaded door. The door starts to open at a differential pressure of 0.2 psi. It is full open at a differential pressure of 0.5 psi
Whay would be interesting to see in simulator in cruise if both packs go off & outflow valves fully closed, how long does it take for depressurization.
Assuming failure of outflow valves to maintain cabin pressurization in whatever situation due to excess leak or failure to pulsate effectively to maintain pressurization the cabin altitude in a steady FL wil continue to rise in accordance with the amount of leak (either via fuselage or partially stuck open outflow valve)
The positive pressure relief valve is a mechanical device. It has an air filter and two ambient pressure sense connections. The air filter supplies cabin pressure to the valve. The ambient pressure connections supply ambient pressure to the valve
When cabin pressure differential at the remote ambient pressure sense port increases to 8.95 psi, the relief valve opens. This permits air to go out of the airplane. If the remote pressure sense port does not function, the relief valve opens when cabin pressure differential at the integral ambient sense port increases to 9.42 psi.
When cabin pressure differential goes below the limit, the positive pressure relief valve closes
The negative pressure relief vent opens when cabin pressure is less than ambient pressure. This prevents negative cabin pressure. Too much negative cabin pressure could damage the fuselage structure.
The negative pressure relief vent is a spring-loaded door. The door starts to open at a differential pressure of 0.2 psi. It is full open at a differential pressure of 0.5 psi
Whay would be interesting to see in simulator in cruise if both packs go off & outflow valves fully closed, how long does it take for depressurization.
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Whay would be interesting to see in simulator in cruise if both packs go off & outflow valves fully closed, how long does it take for depressurization.
Cabin leak rates / decompression
"747-100 series UK CAA Certificate of Airworthiness Air Test pressurisation leak rate test, para 7.2.6. At 30,000ft, I quote:-
'With normal cabin diff. put GALLEY/LAV FAN ON and switch all A/C packs to OFF. Record leak rate over 1 minute; 750' maximum. Restore normal pressurisation."
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Using a B.747 as an example ; Both outflows operate independently of each other and the chances of both of them failing completely are almost zero . Don't forget , they can also be toggled manually via both AC & DC motors As BAeng. says , in the crz they are just 'pulsing' and are almost closed anyway.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
.
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Depends...
Take the 737 - the acceptable leakage rate is for a 1.5 PSID drop in 100 seconds. Depending on the age of the aircraft and how many leaks it has this can be difficult to meet.
For a 777 - the same acceptable rate for a 1.5 PSID drop must be 240 seconds.
I know from experience that if you deliberately manually fully close the outflow valve in a 737-200 at 30,000 ft at the beginning of a rapid descent due to uncommanded cabin rate of climb, (not a rapid depressurisation). the cabin altitude went from 11,000 ft to around 3000 ft in about one minute. The ear pain was almost intolerable. The initial problem turned out to be a defective pressurisation controller. The captain (self) over-reacted to the cabin warning horn thinking it was an impending rapid depressurisation and told the F/O to close the outflow valve when it transpired that was unnecessary under the circumstances.
All we had was an uncommand pressurisation change resulting in a cabin rate of climb of approx. 1500 FPM. All our previous training in the simulator had been rapid depressurisation after the cabin altitude warning had sounded and we were hot wired to react instantly instead of taking out time to don masks and then methodically trouble shoot the reason for the warning. We never practiced trying to get the cabin under control first. Not making excuses for the stuff up but I believe our previous training in the simulator could have accented the difference in crew actions between a rapid depressurisation (and emergency descent scenario) and a mere uncommanded cabin rate of climb. One thing for sure and that is you don't play around with the outflow valve when airborne because cabin rate of change of pressure is unbelievably fast as will be passengers complaints of pain to their ears.
All we had was an uncommand pressurisation change resulting in a cabin rate of climb of approx. 1500 FPM. All our previous training in the simulator had been rapid depressurisation after the cabin altitude warning had sounded and we were hot wired to react instantly instead of taking out time to don masks and then methodically trouble shoot the reason for the warning. We never practiced trying to get the cabin under control first. Not making excuses for the stuff up but I believe our previous training in the simulator could have accented the difference in crew actions between a rapid depressurisation (and emergency descent scenario) and a mere uncommanded cabin rate of climb. One thing for sure and that is you don't play around with the outflow valve when airborne because cabin rate of change of pressure is unbelievably fast as will be passengers complaints of pain to their ears.
